Lung ventilation equipment of the kind with which the invention is concerned comprises means, often an oscillator powered by the breathable gas from a suitable high pressure source, adapted to deliver breathable gas pulses of appropriate volume and at an appropriate repetition rate to a valve associated with a face mask or intratracheal tube: this valve is usually termed "the patient valve" and has the function of switching a connection to the patient's respiratory passage-- i.e. a connection to the mask or intratracheal tube, as the case may be-- alternatively to an inhalation port connected to the gas pulse source or oscillator, and to an exhalation port. These patient valves are of various designs and have the primary function of responding to the arrival of a breathable gas pulse from the gas pulse source by directing the breathable gas to the patient's respiratory passages and then changing over so as to permit the patient to exhale via the exhalation port. Often these valves are arranged to permit spontaneous breathing by the patient to occur without constraint.
Most patient valves may be broadly classified in one of two groups depending upon the characteristics of the gas pulses delivered by the gas pulse source or oscillator with which they are to be used. Thus there are the so-called "low-pressure" patient valves, mainly intended to be connected to the gas pulse source through large bore connections and to handle relatively large tidal volumes of gas delivered at relatively low pressures by the pulse source. There are, on the other hand, so-called "high-pressure" patient valves adapted for use with pulse sources that deliver pulses of breathable gas at relatively high pressure, the gas being expanded in passing the patient valve so as to be delivered at the appropriate pressure to the patient's respiratory passages. Because of the higher pressures available to overcome flow path resistance and the smaller volumes of gas, at the higher pressures, to be transmitted from the pulse source to a high-pressure patient valve, relatively small bore tubing can be used between the pulse source and the patient valve and the operation of such a patient valve is inherently more reliable owing to the larger forces available from the high pressure gas to accomplish movement of its moving parts.
In the case of constant-flow equipment having a low-pressure pulse source and a low-pressure patient valve the large bore connections needed between the pulse source and patient valve to handle the required tidal flow volumes with minimum pressure drop and flow restriction increase the pneumatic compliance of the system downstream of the pulse source and prevent the generation, at the patient, of the ideal pressure waveforms for effective lung ventilation.
In the case of constant-flow equipment having a high-pressure pulse source and high-pressure patient valve connected by relatively small bore tubing there is obtained the advantages of compactness and reduced pneumatic compliance downstream of the pulse source. However, it has hitherto been accepted that the length and other physical characteristics of the small bore tubing must affect the gas flow rate and that the latter will also be influenced by manufacturing tolerances in the patient valve; for this reason such equipment is always calibrated and adjusted as a complete system including the patient valve and connecting tubing that are selected to be used with the pulse source of the equipment.
An object of the present invention is to provide a lung ventilating equipment that has the above-discussed advantages of equipment using small bore connections but avoids the disadvantages usually associated therewith.